Prostaglandin F 2 alpha (PGF2 alpha) will be given to bulls, in doses too small to act systemically, into a spermatic artery, into a carotid or into the cerebral-spinal fluid to determine whether LH release after prostaglandin treatment represents an action directly on the pituitary/hypothalamus or indirectly through modification of gonadal steroids. Cattle will be pretreated with gonadotropin releasing hormone (GnRH) and then given PGF2 alpha to determine whether LH release after PGF2 alpha treatment represents an action on GnRH release. Preliminary experiments will be conducted to determine whether pretreatment with somatostatin will inhibit growth hormone release, and pretreatment with ergot alkaloids will inhibit prolactin release in response to PGF2 alpha. Cattle will be treated with inhibitors of protaglandin synthesis to determine whether such treatments block normal episodic surges of pituitary hormones. BIBLIOGRAPHIC REFERENCES: Hafs, H.D., T.M. Louis, J.N. Stellflug, E.M. Convey and J.H. Britt. Blood LH after PGF2 alpha in diestrous and ovariectomized cattle. Prostaglandins 10: 1001-1009, 1975. Kiser, T.E., H.D. Hafs and W.D. Oxender. Increased blood LH and testosterone after administration of prostaglandin F2 alpha in bulls. Prostaglandins 11: in press, 1976.